Some Factors That Influence The Ecology Of Acacia Xenihophloee Benth. (fever Tree) In Lake Nakuru National Park, Kenya

Abstract

Lake Nakuru National Park is situated in Kenya's Great Rift Valley and occupies about 42 km2 at the lowest point in a wider catchment basin of approximately 1800 km'. Acacia xanthophloea (Fever tree) woodland dominates the major habitat along the park's rivers, lakeshore and flood plains. This woodland provides habitat for a range of large herbivores, mainly Rothschild's Giraffe (Giraffa camelopardalis rothschildi), Black Rhinoceros (Diceros bicornis), Olive Baboon (Papio anubis), Vervet Monkey (Cercopithecus pygerythrus) and several other mammals and insects. In addition the woodland also contributes to the park's aesthetic value and as a tourist attraction. Acacia xanthophloea trees have been undergoing an unexplained die back in the park due to unknown causes over the last few decades. This study investigated the environmental factors that influence the ecology of Acacia xanthophloea trees and armed at identifying the causes of the die-back. Four different sites within A. xanthophloea habitat were selected for the study structure and composition of the woodland by using rectangular plots. Within each plot, density, height, canopy cover and stem basal area of woody species were estimated. Sampling was conducted seasonally both during the rainy and dry seasons of the year. Data were collected quarterly between May 2002 to May 2004. The study on the impacts of intense browsing pressure by large herbivores on the growth of A. xanthophloea trees was carried out using a fenced plot. This was compared with an adjacent unfenced plot. Soil, water, bark and leaf samples of A. xanthophloea trees were also collected. Vegetation profile diagrams were constructed from the transect data collected from vanous woodland habitats. Results clearly indicated that the four study sites were dominated by Acacia xanthophloea trees. Other woody species include, Grewia similis, Rhus natalensis, Lantana camara, Abutilon mauritianum, May tenus heterophylla were also encountered.

Significant differences were found in height growth, canopy cover and stem diameter at breast height (DBH) of Acacia trees in the fenced plots as compared with the unfenced plots both in the wet and the dry seasons. The growth in height and canopy of A. xanthophloea trees was found to be significantly higher in the fenced (un-browsed) plots than in the unfenced (browsed) plots. However, growth in DBH of Acacia trees was found to be significantly higher in the unfenced plots than in the fenced plots. Six essential and non-essential elements including nitrogen (N), copper (Cu), zinc (Zn), chromium (Cr), cadmium (Cd) and lead (Pb) in the soil, water and plant samples were analyzed using Atomic Absorption Spectrophotometry (AAS) technique for Cu, Zn, Cr, Cd and Pb, whereas N was analyzed using micro Kjeldhal method. Measurement of stable isotope composition of carbon and nitrogen in the soil and plant samples were made with the use of Isotope Ratio Mass Spectrometer (IRMS). Analysis of variance and student's t-test revealed that there was a significant (p < 0.05) seasonal trend in variation in the soil and water mean physico-chemical parameters and elemental concentrations. Also, significant (p < 0.05) differences were observed in the mean elemental concentration of the bark and leaves of A. xanthophloea trees obtained from different sites depending on a particular season. This study concluded that Lake Nakuru is becoming increasingly degraded due to" the expansion of agriculture and industrialization in its environments. Its riparian and terrestrial vegetation mainly consist of Acacia xanthophloea woodland that is under threat from domestic, industrial (toxic heavy metals) and agricultural (pesticides) wastes. Pollutants enter through the sewerages, rivers, landfills, storm drainage, runoffs and underground water pollution. Therefore, its conservation requires a thorough understanding of how the lake ecosystem. and its riparian and terrestrial areas function under the impact of human activities.